Small vehicles, such as lawn mowers, lawn and garden tractors, snow throwers, and the like, include an energy source, such as an internal combustion engine, which is used to provide power for rotatably driving an axle which is coupled to wheels which are to be rotatably driven. Most typically, the energy source operates at a single, rotary mechanical speed. Yet, for practical reasons, the axle needs to be able to be rotatably driven at a variety of forward, reverse, and/or neutral speeds. Accordingly, such vehicles may incorporate a transaxle which is used to convert the single speed, rotary mechanical motion of the energy source into a variety of output speeds.
Generally, a transaxle comprises a transaxle input shaft which is operationally coupled to the energy source, a transaxle output shaft, e.g., an axle, which is operationally coupled to the items, e.g., wheels, which are to be rotatably driven, and transaxle componentry which operationally couples the transaxle input shaft to the transaxle output shaft. It is the transaxle componentry which converts the single speed, rotary mechanical motion received from the energy source into a variety of output speeds for rotatably driving the output shaft.
Variable speed transaxles have been developed which control output speed through a single lever. In a typical mode of operation, the lever is moved forward to move the vehicle in the forward direction or pulled backward to move the vehicle in the reverse direction. The farther forward or backward the lever is displaced, the faster the vehicle travels in the corresponding direction.
One form of variable speed transaxle now in use includes a hydrostatic transmission of the type including a hydrostatic pump fluidly coupled to a hydrostatic motor. The hydrostatic pump converts rotary mechanical motion of an input shaft into controllably variable fluid motion. The motor converts such fluid motion back into variable rotary mechanical motion. The rotary mechanical output of the motor is then transferred to the axle by componentry such as a mechanical gear train. The rotational speed outputted by the motor and transmitted to the axle depends, in substantial part, upon the flow rate of the fluid being pumped.
Radial piston pumps and radial piston motors have both been widely used in hydrostatic transmissions of previously known transaxles. A radial piston pump and motor each generally include a rotary cylinder block including radially disposed cylinder bores. The bores house pistons which are capable of reciprocating motion within the bores. The rotary cylinder block is rotatably mounted inside a track ring. The heads of the pistons are coupled to the track ring by slippers which travel around the inside of the track ring as the rotary cylinder block rotates. The track ring is disposed eccentrically around the rotary cylinder block so that the pistons are pulled out of the bores on one side of the rotation cycle (i.e., the suction part of the cycle) and are driven into the bores on the other side of the rotation cycle (i.e., the discharge part of the cycle).
In operation, the rotary cylinder block of the pump is rotatably driven by an input shaft, thus causing the pump pistons to reciprocate in the pump cylinder bores. Such reciprocation creates a pumping action for transporting hydrostatic fluid to and from the motor which is fluidly coupled to the pump. The transport of the fluid creates a pressurized fluid flow that drives the motor pistons. This, in turn, causes the motor rotary cylinder block to rotate within the motor track ring. Rotation of the motor rotary cylinder block rotatably drives a motor output shaft. The track ring of the pump is pivotable, which allows the operator to vary the eccentricity of the track ring relative to the pump rotary cylinder block. Generally, increased eccentricity increases the length of the pump piston stroke, and a longer piston stroke corresponds to higher output speeds. Thus, by pivoting the track ring, the operator controls output speed. The pump track ring can also be pivoted in two directions away from a neutral setting. One direction corresponds to a forward mode of operation, and the other corresponds to a reverse mode of operation. Whereas the pump track ring is pivotable, allowing the operator to control output speed and direction, the motor track ring is most typically eccentrically fixed relative to the motor rotor cylinder block. U.S. Pat. No. 5,182,966 (von Kaler), as one example, describes a particularly effective and reliable hydrostatic transmission for a transaxle in which the transmission includes a radial piston pump fluidly coupled to a radial piston motor.
A radial piston pump is one of the most efficient and effective ways for converting rotary mechanical motion into fluid motion. However, a radial piston motor is somewhat less efficient at converting fluid motion back into rotary mechanical motion. Accordingly, it would be desirable to improve the efficiency of the motor component of a hydrostatic transmission of the type including a radial piston pump so that the overall efficiency of the transmission could be improved.
In previously known radial piston pump and motor assemblies, the piston heads are typically coupled to the slippers by a direct mechanical linkage such as rivets, pins, and the like. Although reliable as far as the operator is concerned, such linkage tends to increase the complexity, parts count, expense, and/or time required for transmission assembly. It would be desirable, therefore, to simplify the manner in which the piston heads are coupled to the slippers.
Radial piston pump and motor assemblies tend to be subject to vibration forces which arise due to the substantial pressure differences between the suction and discharge sides of the rotary cylinder block. For example, the discharge side of a rotary cylinder block of a radial piston pump may be typically characterized by a discharge pressure on the order of 1500 psi, whereas the suction side of the rotary cylinder block may be characterized by a suction pressure on the order of -5 psi. When the rotary cylinder block rotates at ordinary rotational speeds, e.g., 1500 to 4000 rpm, such pressure differences tends to set up vibrations that are not only noisy, but may also be severe enough such that the vibrations could even damage the transmission if not controlled properly. Previously, transaxles have employed mechanical means, e.g., clamps, to help hold a radial piston assembly in proper position and thereby attempt to overcome vibrations by physical clamping force. Such techniques, however, do not eliminate the magnitude of the vibration forces, thus requiring the mechanical means to absorb and control the full magnitude of such forces. Accordingly, there is a need to provide such transmissions with a way to reduce the magnitude of the vibration forces in order to reduce, and even eliminate, the demands placed upon the mechanical means used to absorb and control such forces.